This invention relates in general to the treating of material with gas or vapor contact, and in particular, to the in-bin drying and conditioning of grain.
The Energy Costs of Drying
Due to uncertainty and lack of understanding, most operators continuously heat the grain and only use cooling to reduce spoilage risk when the grain is dry. The energy cost of this conventional practice is huge. It can exceed all the other grain production costs combined (except fertilizer). In 1978, 75% of a seven billion-bushel corn crop required nearly 875 million gallons of LP gas for drying. Today""s corn crops exceed 9 billion bushels and require 90% artificial drying. In addition, millions of dollars are lost each year because of grain being cracked from poorly controlled drying.
The basic approach to drying has remained essentially the same since the 1840""s: blowing heated air through the grain followed by ventilation just for cooling. It has been assumed that heating the grain was the only method for accelerating drying. In those instances where researchers did cool their grain to aid drying, they only did so as a one-time procedure.
Relative humidity (RH) and its companionsxe2x80x94Equilibrium Moisture Content (EMC) and Equilibrium Relative Humidity (ERH)xe2x80x94have remained the principal control variables. On occasion grain moisture sampling is used for direct feedback. On the surface, these concepts appear quite reasonable; however, our drying research has uncovered the following:
1) Relative Humidity, as a control in itself, is fundamentally flawed.
2) Grain Moisture feedback, as commonly used, is also flawed.
3) The use of constant heat does not produce drying optimization.
4) The standard methods for solar heating are often counter-productive.
5) The xe2x80x98wet warmingxe2x80x99 of grain during warm fronts followed by the planned use of cooling during the nights of cold fronts, is much more efficient than continuos heating.
It is the objective of this art to advance methods for drying materials in general, and grain in particular, with greater certainty, control, quality, and economy.
The Theory, Methods, and Means to do so:
The basic problem of the prior art is an inadequate expression of the nature of drying.
It was known since the 1930""s and proven in 1940 that the driving force for drying is differences in vapor pressure. However, the early researchers directly measured grain moisture and indirectly described these vapor pressure differences in terms of relative humidity (RH) and equilibrium limits. At first this was due to the lack of the needed sensors. However, even when the sensors did become available, the use of comparative measures continued. Without the ability to quantify relationships, optimization is not obtainable.
Theory of Differential Vapor Pressure (dPv):
This instant art advances a method of indirectly measuring the material and directly analyzing dPv""s. As I will show, this allows integration of temperature and moisture relationships and provides a method for quantifying the changes. Out of this emerges a better understanding of the needed sensing and sensor placement, data formatting, and the subsequent opportunities for optimization.
Direct computation of differential vapor pressure opens a new method of analyzing drying changes. While these concepts are not obvious and intuitive at the start, the underlying dynamics are simple extensions of classic concepts. With such analysis, the present art can open a whole new paradigm of drying.
Logic.
The key to understanding drying and sensor placement is to understand the role of pressure in drying, particularly that of vapor pressure.
A: Pressure is the only manipulable force for moving fluids.
B: Water, water vapor, and air are fluids, (fluidxe2x80x94Greek, to flow).
C: Drying only occurs if the pressure for the vapor to leave the material is greater than for it to re-enter or remain.
D: The Ideal Gas law is the foundation for understanding Pressure.
In Drying and Storage of Grains and Oilseeds, Donald B. Brooker, Fred W. Bakker-Arkema, C. W. Hall, published by Van Nostrand Reinhold, New York, (633.1046/b791ds), write xe2x80x9cUnder the conditions at which grain drying takes place, the Ideal Gas Law expresses accurately the relationship between the pressure, temperature, and volume for the dry air and associated water vaporxe2x80x9d.
The Ideal Gas Law is traditionally written as: PV=nRT.
P=pressure; V=volume; n=moles; R=gas constant; and T=temperature.
With M=mass of moisture replacing the chemical term n=moles, we can rewrite the traditional expression in terms of pressure:
P=M/V*T*R,
If we set V=1 cu ft and regard it as a constant we can omit the V term. Likewise, R being a constant, we can for convenience, omit it also.
Results: Pxcx9cM*T: Pressure is proportional to a product of Mass (of the moisture) and Temperature (of the moisture).
Proportional=xcx9c, for sake of writing with this word processor.
To express vapor pressure differences mathematically requires two opposing pressure equations.
P(material)xe2x88x92P (air)
Omitting the P term on each side of two pressure equations and substituting the mass and temperature variables for air (a) and grain (g) in this instance leaves the following:
Basic Model:
dPv=[(Mg*Tg)xe2x88x92(Ma*Ta)]
with M=actual moisture per unit volume and T=temperature.
Key: Each vapor pressure difference is dependent on two opposing multivariate expressions. Four values are needed to precisely define each relationship.
Inductive conclusions of the symbolic model.
1) Looking at the symbolic model, (Mg*Tg)xe2x88x92(Ma*Ta), it is possible to see that constant or continual heating will not produce optimization. When the temperatures have reached equilibrium, the only remaining driving force is the differences in moisture concentrations. While this could be large, it can not be as large as a xe2x80x98loweredxe2x80x99 ambient temperature or moisture or both. When the grain and air temperatures have reached equilibrium, the use of naturally cold or chilled air will be more efficient than constant heat.
2) In addition, it is possible to see why drying can be facilitated by the deliberate ventilation of warmer grain with colder, drier air. When air is selected (or conditioned) for low ambient temperature and moisture, its ambient vapor pressure is low. Since drying is driven by difference, this xe2x80x98loweringxe2x80x99 of the outside vapor pressure increases the drying potential. Also, the effect of the lower moisture can be dramatic. A two-degree drop in incoming dew point can have the same effect as a twenty-degree rise in grain temperature. Moreover, this increased potential for drying comes minus the cost of expensive fossil fuels and the penalty of the added vapor from the combustion of those fossil gases.
Hidden bias: The Grain as just an Object.
In the past, an almost unconscious attitude developed that grain is just an object to be treated and that all the drying heat comes form the air. It""s our finding that having the grain xe2x80x98participatexe2x80x99 in the drying not only can lead to greater efficiency, but also higher quality. This participation is in two forms.
First, because grain has an inherently higher specific heat than air, it can act as a solar collector and heat storage means. Instead of having to build solar collectors and thermal storage means external to the bin, we simply xe2x80x98storexe2x80x99 heat from warm fronts (or auxiliary heating) in the grain. Conventional use of solar heating can actually be counter-productive. When the air is heated before entering the bin, it can be made xe2x80x98tooxe2x80x99 dry. The heated air can be so low in moisture that it overdryes the bottom layers and then transports that moisture to the upper layers leaving them too wet.
Second, when outside air is colder and dryer that the grain, running the fans makes the grain the xe2x80x98heaterxe2x80x99 of that air. This cold, dry air already has a naturally low vapor pressure. However, as the incoming cold air is heated, it expands. This expansion makes this air relatively even dryer. Also, when the grain is the source of heat for the incoming air, it is providing both the heat for the sensible change and for the latent heat for evaporation. The effect can be a three-fold increase in efficiency for a bin, a 16-fold increase for a layer.
In addition, the air is never heated above the grain""s temperature or made xe2x80x98wetterxe2x80x99 than the grain""s moisture. Therefore, as this air then exits, all the moisture that evaporated remains in the exiting air and does not re-condense in the upper grain layers. In reverse, if cold grain were being intensely heated, the grain at the bottom would dry, but as the hot, moist air rose through the cold upper layers, much of it could re-condense and have to be re-evaporated, maybe several time over.
When the earlier researchers began studying drying they did not have access to remote monitoring and microprocessing. However, they did have access to absorptive RH sensors. Because of this, they focused on RH and made assumptions to ease calculations. When RH is the focus, the means for effecting drying center on heat or selecting air with certain RH levels. Cooling the grain would be something done only when the drying was complete. When cooling was used for drying, the focus was a one time cooling of grain emptied from hi-temp dryers.
A differential vapor pressure model in combination with current technology changes this thinking. The technology enables remote sensing and the model directs the data be sensed as, or converted to, pairs of independent temperatures and moistures, preferably as Dry Bulb (DB) and Dew Point (DP) values. The newly developed, rugged, and cost efficient RH sensors can be used, however their values would be used together with their paired temperatures to calculate dew point values. Even without quantification, the simple plotting of DB and DP values changing over time presents an intuitive view of drying progress (see FIGS. 1-a and 1-d). However, the model aids in forecasting and adaptive control, means.
Indirect Material Measurement.
In the technical appendix I list in detail the factors of genetic varieties, physical and chemical variations, different growth histories, and harvest needs which can affect the grain""s drying rates. The prior art is very limited in dealing with this diversity (see appendix of prior art for the critiques in detail). However, by incorporating the older concepts of equilibrium with paired sensing as Dry Bulb (DB) and Dew Point (DP) values, we can effect a new approach of data analysis. We operationally effect a functional integration of the grain""s variations as independent values comparable to all the other data points. Weather can be sensed or converted to a series of incoming Dry Bulb and Dew Points. Thus different vertical and horizontal grid points within the bin, the air inlet/outlet values and distant incoming weather patterns can all be formed into a common data format.
With a common data format and bin data (mass of grain, airflow, etc.), an algorithm can be developed to quantify the pounds of water removed (WR) between any two points. With quantified values, incoming weather potentials can be evaluated with regard to the grain for start up times. Once running, a comparison of the inlet and exit values helps determine optimum times for stopping the fans. By being able to progressively monitor both the internal changes during drying and the incoming weather, adaptive feedback control is possible.
In addition, by being able to quantify the amount of water being removed, the enthalpy of the latent heat (QL) can be calculated. With QL, a Heat Balance calculation is now possible. With a Heat Balance, the time to temperature equilibrium (Te) is derivable. With WR as a measure of mass exchange potential and Te as a time calculation, we can begin evaluating the incoming weather in terms needed fan times and costs. Once the awareness of the potential savings from optimization can be made visible to the bottom line, the adoption of these techniques will soar.